The invention relates to the production of video disks from an original matrix having a relief, whose transfer by moulding uses a resin. The information is stored in the form of surface hollows or protruberances along a track materializing the time variation of the information-varrying signals.
The moulding processes are particularly suitable for the production of small numbers. They consist of placing between a matrix and a substrate, an intermediate moulding agent which, after solidification, supplies a countertype of the impression carried by the matrix. This moulding process introduces the idea of complementarity between the matrix and the disk, because the hollows of the one correspond to the reliefs of the other and vice versa.
The original disk carries successive tracks etched e.g. by thermal ablation in accordance with micro-pedestals. A copy disk is obtained, whose surface formed by the hardened moulding agent layer reproduces the same relief in negative, i.e. on the basis of the chosen examples microreliefs. To obviate the placing of a parting agent layer between the original disk and the hardenable substance, which is liable to be locally prejudicial to the micro-relief quality, a resin is used, which can easily be spread and which when hardened does not adhere to the original disk, but does adhere to the substrate serving as a support for it.
One production method consists of depositing the resin close to the centre of the matrix. A flexible substrate is curved and applied to the resin and is then progressively flattened against the matrix in order to displace the resin towards the periphery of the substrate. The resin is then polymerized, e.g. by ultraviolet radiation. The curve or contour can be obtained with the aid of a system comprising a support ring supporting the substrate on its periphery and a vertically movable, elastically deformable cushion which makes it possible to exert a progressive pressure on the substrate, as described in French Patent Application No. 79.22300 filed by the applicant.
As a result of this process, the resin covers the surface of the matrix and any excess resin forms a flange on the outer edge of the matrix. Towards the centre of the device, the excess resin can flow back into the space between the centering projection of this device and the substrate. If the resin excesses are polymerized, they can constitute attachment points making mould removal more difficult or destroy the central bore of the substrate. Furthermore, all or part of the irregularly shaped flange remaining attached to the substrate can be prejudicial to the subsequent use of the disk.
If polymerization of the edges of the matrix is avoided, e.g. by using removable covers, the remainder of the unpolymerized resin must then be cleaned and may make the recording area dirty. The cleaning of polymerized or unpolymerized resin excesses is in any case a tedious operation.
In addition, numerous applications require the protection of the recorded area by means of a cover fixed to the substrate by joints located outside the recorded area. A plastic cover can, for example, be stuck or welded to the substrate. However, it is difficult to produce these joints, due to the presence of a resin layer on one or other of the surfaces to be assembled.
In order to obviate these disadvantages, it is proposed to produce information-carrying disks in such a way that all the resin used is polymerized and remains on the substrate after mould removal, without forming a resin flange with uncontrolled dimensions. This can be obtained by small stop areas collecting the excess resin.